Sid, a goofy but good-natured sloth, is left behind by his family and the herds of mammals journeying to the south. He meets Manny, a cynical and loner woolly mammoth who is travelling to the north, and decides to follow him, much to Manny's disdain. Along the way, the pair come across a woman who has jumped down the nearby waterfall in a desperate bid to protect her child after their camp was attacked by saber-toothed tigers. She silently pushes the infant towards Manny and Sid before disappearing into the water. The two animals decide to search for the father and return the baby to him. However, the villainous Soto, leader of the sabers, instructs his second in command, Diego, to bring the baby to him so he can exact his revenge on the child's father, who frequently kills sabers with his tribe for their furs and meat. Diego comes across Manny and Sid and, since they refuse to just give him the child, he pretends to lead them towards the baby's tribe, when really Soto and the rest of the pack wait in an icy mountain range to ambush them, intending on killing Manny and stealing the child. During their journey however, the trio bond over the infant and Diego must ultimately decide where his loyalties lie.
